Paper from pitch based organic fibers

ABSTRACT

Bonded fibrous articles are made from mixtures of fusible and infusible pitch-novolac fibers, as well as similar mixtures of fusible and infusible novolac fibers. The fibrous mixtures are formed into the desired shapes and the fusible fibers are cured, forming firm bonds with the infusible fibers during the curing step. The resulting fibrous articles show excellent dimensional stability as well as chemical and thermal resistance. The fibers may be formed into flat sheet-like products which are suitable as thermal and electrical insulators and show good resistance against chemical attack.

United States Patent 1191 Lin et a1.

[ PAPER FROM PITCH BASED ORGANIC FIBERS [75] Inventors: Ruey Y. Lin, Williamsville; Hari N.

Murty, Grand Island; Anthony J. Pietrantuone, Tonawanda, all of NY.

[73] Assignee: The Carborundum Company,

Niagara Falls, NY.

[22] Filed: Dec. 7, 1972 [21] Appl. No.: 313,007

[52] U.S. Cl 162/146, 136/146, 136/148, 162/138,162/157 R [51] Int. Cl D2lh 5/12 [58] Field of Search 162/138, 146, 157 R, 152, 162/165, 171; 423/447; 260/28; 264/29;

[56] References Cited UNITED STATES PATENTS 3,265,557 8/1966 Defries et a1. 162/146 X 3,301,803 l/l967 Schick 260/28 3,360,462 12/1967 Littler 260/28 X 3,367,851 2/1968 Filreis et a1. 162/138 X 3,563,802 2/1971 Ogden 136/146 X [11] 3,821,074 June 28, 1974 3,595,946 7/1971 .100 et a1. 264/29 3,639,953 2/1972 Kimura et al. 264/29 3,694,310 9/1972 Emanuelson et a1 162/157 R FOREIGN PATENTS OR APPLICATIONS 1,080,866 1967 Great Britain 1,091,890 1967 Great Britain Primary Examiner-Robert L. Lindsay, Jr. Assistant Examiner-William F. Smith Attorney, Agent, or Firm-The Carborundum Company [57] ABSTRACT 14 Claims, No Drawings 1 PAPER FROM PITCH BASED ORGANIC FIBERS BACKGROUND OF THE INVENTION The present invention relates to fibrous articles made from fibers comprising novolacs, as well as fibers comprising pyrogenous residues, such as pitch, combined with appropriate novolacs. The fibers may be either fusible or infusible, depending on the method of production and the fibrous articles made therefrom may take the form of mats or sheet-like products.

Methods of the prior art for preparing infusible pitch fibers, such as described in US. Pat. No. 3,595,946, generally require extensive treatment of the pitch prior to spinning a fusible fiber and subsequently oxidizing the fusible fiber with ozone and air for a long period of time to form an infusiblesheath on the fiber. US. Pat. No. 3,595,946 describes a treatment suited for coal tar pitch while British specification Pat. No. 1,091,890 describes a treatment suited for petroleum pitch. Prior art processes have concentrated on oxidizing the fusible pitch fiber to render'it infusible and thus require complicated or time consuming processes, which often fail to give fibers of uniform strength or dimensions. The production of mats or sheet-like fibrous products consisting essentially of synthetic resin fibers has met with many difficulties in the past. Synthetic fibers in general are characterized by their smoothness and inability to hydrate in aqueous suspension. They show little tendency tocling to one another when laiddown in mats or thin layers. Satisfactory bonding between the fibers of a fibrous article or mat has therefore been difficult to achieve, resulting in poor cohesion and laclt of dimensional stability in the fibrous article.

In conventional processes for preparing bonded fibrous articles, a mat of fibers is coated or sprayed with a liquid binder which is then cured by heating with a liquid or gaseous curing agent. The disadvantages of this process lie in the formation of a surface film which renders the product impervious even though the overall bulk density of the product is generally low. The spraying of the binder leads to a non-uniform bonding which is reflected in poor mechanical properties and gives a product which is either impervious or of non-uniform porosity. If the fiber material is prepared from a mixture of resins, then a binder of identical composition must be used, otherwise a fiber-binder system having varying chemical stabilities results. Since synthetic fibers possessmany useful properties, such as heat and chemical resistance, an improved method of producing bonded fibrous articles from heat and chemical resistant synthetic fibers is highly desirable.

SUMMARY OF THE INVENTION Bonded fibrous articles are made from a mixture of fusible and infusible synthetic fibers, the fibers being synthesized either from novolacs or from a pyrogenous residue, such as pitch, combined with a novolac. The process comprises suspending the fibrous mixture in a fluid, depositing the fibers upon a permeable support, removing the fiuid and adding a curing agent to the fibers, and then compresssing the fibers and heating to form the bonded article. The fiber mixture may comprise about to 40 wt. percent of fusible fibers combined with about 60 to 90 wt. percent of infusible fibers, the curing agent being an aldehyde or an amine combined with a catalyst. The pressure applied during curing may vary from about 0 to 5 psi, depending on the density desired in the bonded article, while the curing temperature may range from about to 200 C. Heating may be continued in the range of about 200 to 400 C, if an article of improvedthermal stability is desired. A preferred fiber mixture comprises about 20 wt. percent fusible fibers and 80 wt. percent infusible fibers, the mixture then deposited on a screen as a flat sheet and cured with hexamethylenetetramine at temperatures ranging from about to 400 C to form a bonded fibrous sheet. Fibrous articles made by this method are firmly bonded during the curing reaction of the fusible fibers. The articles may be porous with average pore diameters in the range of l to 50 microns and may be in the form of flat sheets with a sheet thickness which may range from about 1 to 250 mils but preferably is from I to 10 mils. Since the method of curing gives a strong bond between fibers and allows the ready control of pore size and product dimensions, the process of the invention overcomes the deficiencies found in prior art processes andproduces a product having superior mechanical strength, combined with outstanding thermal andchemical resistance. Porosity of the product may be readily controlled, since the uncured fibers are the'binders and bonding takes place only at the contact points between the cured and uncured fibers. Since both types of fibers are produced from the same batch,the resulting bonds are chemically compatible, even when the fibers are produced from'an initial mixture of resins.

DETAILED DESCRlFTlON The startingpyrogenous residues that may be used in the present invention include a variety of pitches such as coal tar pitches, pitches obtained by distillation of oils, petroleum pitches, pyrogenous asphalts, and a va riety of pitch-like substances produced as by-products of various industrial processes such as distillation residues. Preferably, the starting pyrogenous residue has a softening point of about 80 C to about 200 C, more preferably from about 100 C to about C. Preferably, the pyrogenous residue has a carbon to hydrogen ratio based on weight percent from about 18 to about 25. The content of aromatic and unsaturated components may vary depending on the source of the raw material pyrogenous residue.

Preferably, pyrogenous residues used as starting materials have a beta-resin content greater than about 5 percent and preferably greater than 10 percent by weight. The beta-resin is the benzene insoluble content of the pyrogenous residue minus the quinoline insoluble content. In making the determination, there are other solvents, such as toluene, which can be substituted for benzene and pyridine which can be substituted for quinoline. The beta-resin portion of the pyrogenous residue is believed to enhance the binding and adhesive qualities thereof. It is believed that a suitable amount of beta-resin contributes to rendering the fusible fiber infusible by a short curing process. The upper limit of the weight percent of beta-resin in the starting pyrogenous residue is not. critical but is gener ally limited by the type of pitch used and process conditions. Most commercially available pitches have a betaresin content less than about 30 percent but pitches with a beta-resin content higher than 45 percent may be used in the present invention.

Generally, commercially available coal tar pitch has a benzene insoluble content of about 20 to about 50 percent by weight and a quinoline insoluble content of about 10 to about 20 percent by weight with a resulting beta-resin content in the range of about 10 to 30 percent. These pitches are suited for using as a starting material in the present invention without further modificatron.

Petroleum pitches and pyrogenous asphalts often have beta-resin contents less than about 5 percent. This is generaly due to a low percentage of benzene insolubles which is generally less than about percent. In such a case, while the fusible fiber of pyrogenous residue and novolac can be rendered infusible by reacting with formaldehyde, the curing process is comparatively slow. Alternatively, it is possible to upgrade the pitch by increasing the beta-resin content. Such upgrading can be done by reacting the pitch or asphalt with an aldehyde and phenolic compound in the presence of an acid catalyst at a temperature sufficiently high to effect condensation between the pitch or asphalt, aldehyde and phenolic compound. Such a method'is described in U.S. Pat. No. 3,301,803 which is incorporated into the present case by reference. The amount of aldehyde and phenolic compound that is employed can vary widely depending on the degree of upgrading necessary. The reaction is carried out at a temperature from about 150 F to about 600 F for a suitable period of time.

The amount of quinoline insolubles in the starting pyrogenous residue should be less than about percent by weight and preferably less than about 10 percent. As the percentage of quinoline insolubles in the starting pyrogenous residue is decreased, the ease of fiberization of the melt is increased and the uniformity of the fibers is enhanced. The most preferred starting pyrogenous residue contains zero or a very low percentage of quinoline insolubles. The quinoline insolubles represent material which is not soluble in the pyrogenous residue at the spinning temperature and which forms an undesirable second phase. Removal of the quinoline insolubles can be accomplished by diluting the pitch in an appropriate solvent and filtering or centrifuging to remove the insolubles. Such a method is described in U.S. Pat. No. 3,595,946. 7

A wide variety of novolac resins may be used as starting materials in the present invention. The term novolac refers to a condensation product of the phenolic compound with formaldehyde, the condensation being carried out in the presence of a catalyst to form a novolac resin wherein there are virtually no methylol groups such as present in resoles and wherein the molecules of the phenolic compounds are linked together by a methylene group. The phenolic compound may be phenol, or phenol wherein one or more of the non-hydroxylic hydrogens are replaced by any of various substituents attached to the benzene ring, a few examples of which are the cresoles, phenyl phenols, 3-5 dialkyl-phenols, chorophenols, resorcinol, hydroquinone, chloroglucinol and the like. The phenolic compound may instead be naphthyl or hydroxyphenathrene or another hydroxyl derivative of a compound having a condensed ring system.

For purposes of the present invention, any fusible novolac which is capable of further polymerization with a suitable aldehyde or amine may be employed for the production of fibers. Stated another way, the novolac molecules must have two or more available sites for 4 further polymerization. Apart from this limitation, any novolac might be employed, including modified novolacs, i.e., those in which a non-phenolic compound is also included in the molecules, such as the diphenyl oxide or bisphenol-A modified phenol formaldehyde novolac. Mixtures of novolacs may be employed or novolacs containing more than one species of phenolic compounds may be employed.

Novolacs generally have a number-average molecular weight in the range from about500 to about 1,200, although an exceptional case in which the molecular weight may be as low as 300 or as high as 2,000 or more may occur. Unmodified phenol formaldehyde novolacs usually have a number-average weight in the. range from about 500 to about 900, most of the commercially available materials falling within this range.

Preferably, novolacs with a molecular weight from about 500 to about 1,200 are employed in the method of the present invention. When a very low molecular weight novolac is used, the temperature at which such novolacs soften and become tacky is usually comparatively low. Therefore, it is necessary to cure the fiberized novolac at a very low temperature to avoid adherence and/or deformation of the fibers. It is usually undesirable to employ such low curing temperatures since the curing rate increases dramatically with the increase in temperature and low curing entails the practical disadvantage of a prolonged curing cycle. It is generally preferred to employ a novolac having a moderately high molecular weight for the type of novolac under consideration to permit curing in a reasonable time without adherence and/or deformation, but to avoid the extreme upper end of the molecular weight range to minimize problems in fiberizing due to gelling.

A mixture of pyrogenous residue and novolac may be formed by any convenient technique such as dry blending or melting the pyrogenous residue and novolac by heating together to form a homogenous mixture. Mixtures containing from about 5 to about 40 percent by weight novolac may be used for preparing the fibers of the present invention. Since the pyrogenous residue is the most economically available component of the mixture, it is preferred to employ less than about 35 percent novolac by weight. It is preferable that the novolac content be at least about 10 percent'and more preferably that it be at least about 25 percent in the mixture so that the spinnability of the fiber is enhanced and the curing time can be sufficiently reduced. Preferably, the mixture consists essentially of the pyrogenous residue and novolac.

The fiberization may be performed by any convenient method such as drawing a continuous filament downwardly from an orifice in the bottom of the vessel containing a molten mixture of pitch and novolac. The filament is wound and collected on a revolving take-up spool mounted below the orifice. The take-up spool also serves to attenuate the filament as it is drawn from the orifice before it cools and solidifies upon contacting the atmosphere between the orifice and the spool. The melt may also be formed into short staple fibers by methods known in the prior art such as blowing the melt through a fiberizing nozzle and collecting the cooled fibers or blowing a thin stream of melt into the path of a hot blast of gas. These methods produce a staple consisting of a multiplicity of fusible uncured pitchnovolac fibers of variable length and diameter. The diameter of the fibers can vary from 0.1 micron to about 300 microns.

When producing a continuous filament having a uniform diameter by melt spinning, preferably the fibers have diameters from about to about 30 microns. The filament diameter depends primarily upon two factors, the drawing rate and the flow rate of the melt through the orifice. The fiber diameter decreases as the drawing is increased and increases as the flow rate of the melt is increased. The flow rate of the melt depends primarily upon the diameter and length of the orifice and the viscosity of the melt, increasing as the orifice diameter is increased, decreasing as the length of the orifice is increased, and increasing as the viscosity of the melt is decreased. An increase of flow rate may also be effected, is desired, by applying pressure to the melt to force it through the orifice.

Curing of the fusible fiber to render it infusible is effected by heating the uncured fusible fiber in a liquid or gaseous formaldehyde environment. It appears that the curing mechanism involves the diffusion of the formaldehyde into the fiber and reaction of the novolac and formaldehyde to bring about polymerization of the novolac and pyrogenous residue mixture. It is preferred to effect curing by heating the uncured fusible fibers in an environment containing paraformaldehyde in the presence of a catalyst. The environment may be gaseous, but is preferably liquid as in a solution of the catalyst and formaldehyde. Liquid is preferred because of the greater rapidity of heat and material transport to the fibers, especially the fibers in the interior portions of a bundle of fibers being cured, and also because higher concentrations of formaldehyde and catalysts may be achieved by employing a solution thereof. Any of a wide variety of acids or bases may be used as the catalysts, any of the mineral acids or bases such as hydrochloric, sulfuric, phosphoric, ammonia hydroxide, potassium hydroxide, sodium hydroxide and organic acids or bases such as oxalic acid, or dimethylamine being particularly suitable.

When a solution is employed for the curing step, water is the choice of solvent although other liquids may be employed provided that they do not adversely affect the fiber and are capable of dissolving the formaldehyde in a solution containing the catalyst. Preferably, the solution contains from about. 12 to about 18 percent formaldehyde. When an acid catalyst is used, it is preferred that the solution contain from about 12 to about 18 percent acid, and when a base catalyst is used, from about 1 to about 10 percent base. Lower concentrations of catalysts or formaldehyde in the solution generally require longer curing times. Higher concentrations of formaldehyde or catalysts do not appear to offer any advantage.

When curing is carried out in a gaseous environment, any gaseous catalyst such as hydrogen chloride or ammonia may be employed. The formaldehyde may conveniently be generated by heating paraformaldehyde. The gaseous atmosphere maycontain as little as about 10 percent formaldehyde up to as much as 99 percent, by volume, and from about 1 percent to about 90 percent, by volume, of the acid. If desired, the atmosphere may also contain a diluent such as nitrogen or other inert gas, but air should be excluded to minimize the possibility of side reactions taking place. In either a gaseous or liquid environment, the rate of curing increases with increasing temperature. It is possible to cure the pitch-novolac fibers at room temperature (25 C) but is highly impractical to do so'because of the time required. in the interest of minimizing the curing time, it is preferred to cure the fibers at the highest temperature at which adherence and/or deformation of these fibers does not occur. In general, the lower the molecular weight of the mixed resin, the lower the temperature at which these occur. Therefore, it is usually preferred not to use extremely low molecular weight resins, thereby avoiding the need for low curing temperature and the attendent slow curing rates.

It is usually desirable to carry out the curing cycle at gradually increasing temperatures. initially, a temperature is employed at which adherence and/or deformation does not occur. At this stage, the outer portion of the fiber begins to cure, forming a shell, and thereupon, the temperature may be raised as necessary to complete the cure, the shell eliminating problems due to fusion which might otherwise occur. The curing time must be sufficiently long to render the uncured fiber infusible. Once such infusibility has been achieved, further curing is unnecessary for purposes of this invention. At a temperature of C, the time is about 10 hours, while at a temperature of about C, the time is about 3 hours. it is generally preferred to carry out the curing be employingan initial room temperature and increasing the temperature to a final curing temperature of about 80 to about 100 C over a period of from about 1 to 3 hours and maintaining the temperature of a final curing temperature for a residence time of about 2 to about 4 hours for a total curing time of from about 3 to about 10 hours.

When a low weight percent of novolac is employed in the pitch-novolac mixture, such as below about 10 percent novolac, it may be desirable to oxidize the fibers after curing. Although the curing step renders the fibers infusible, if a low weight percent of novolac is employed or if the diameter of the fiber is greater than about 20 microns, the fiber will smoke when subjected to a flame. It is believed that the additional steps of oxidizing promotes the formation of cross links whereby high polymer carbon material of insoluble and unmeltable characteristics is further produced. The oxidizing of the pitch-novolac fibers is performed by heating the cured fiber in air or other oxidizing atmosphere at a temperature of about 200 C to about 300 C. Preferably, the fibers are heated in air from about room temperature (25 C) up to a final temperature in the range of from about 200 C to about 300 C, the temperature being continually increased at the rate of from about 25 C per hour to about 100 C per hour, and continuing to heat in the air atmosphere at the final temperature for about 5 to about 60 minutes or longer.

The methods of fiber formation, both fusible and infusible, have been described at considerable length, since the mechanism of curing the fusible fiber is essential to the proper bonding of the fibrous articles of the invention. The process for making these articles comprises the steps of preparing a mixture of fusible and infusible fibers and suspending this mixture in a suitable fluid. The mixture is then deposited upon a permeable support and the fluid removed, followed by the addition of a curing agent to the fibrous mass. This mass is then compressed and heated, during which time the fusible fibers are cured and become infusible. During the curing step, the fusible fibers bond securely to the infusible fibers which are already present in the fibrous mass, thereby forming strong infusible interfiber bonds throughout the mass.

The composition of the fibrous articles of the invention may be varied over a wide range, depending on the proportions of fusible and infusible fibers used. The mixture of fibers may comprise from about 10 to about 40 percent of fusible fibers and from about 60 to about 90 percent of infusible fibers. Throughout the following description and claims, percentages will be given by weight, unless otherwise specified. A preferred fibrous mixture comprises about 20 percent fusible fibers and about 80 percent infusible fibers. The fibrous mixture may be suspended in any suitable fluid, either gaseous or liquid. Water is the preferred liquid, but others such as alcohols, esters, or hydrocarbon solvents may be used. While not essential, the liquid may contain small amounts of wetting agents or binders such as polyvinyl alcohol or other organic wetting agents or binders which are soluble in the liquid, to promote the proper dispersion of the fibers and to hold them temporarily in place while the fibrous mass is being formed. As an alternate procedure, the fibers may be suspended in a current of gas such as air or nitrogen. The fibrous mixture is then deposited upon a permeable support, such as a screen or perforated plate, and the suspending fluid removed to leave a mat or sheet-like deposit of the mixed fibers. A preferred method for depositing the fibers is to feed the aqueous slurry to a screen such as used in a Fourdrinier papermaking machine, removing the water by suction to form a sheet-like deposit of the fibers, and removing the damp fibrous sheet for further processing.

After the fibrous mat or sheet is formed, a curing agent is 'added to it, which may be any of those previously described and may be liquid or gaseous; a preferred curing agent is hexamethylene tetramine. The amount of curing agent may vary,'depending on the proportion of fusible fibers present in the fibrous mixture. After the addition of the curing agent, the fibrous mat is compressed and heated to cure and simultaneously bond the fusible fibers to the infusible fibers at their cross over points. Only moderate pressures in the range of about to psi are required at this step, with curing temperatures in the range of about 90 to 200 C, as previously described. The pressures described are comparable to those applied to the sheet on the drying rolls of a Fourdrinier paper machine. The fibrous article, as bonded by the curing process described, is infusible and has excellent dimensional stability, combined with good chemical and thermal resistance. Thermal resistance may be enhanced by further heating the article in air in a range of about 200 to 400 C, as previously described.

While the bonded fibrous articles of the invention may be impermeable, depending on fiber composition and pressures applied during curing, the preferred type of article is one which is porous, the pores having average diameters in the range of about 1 to 50 microns. The articles may be formed in any shape; however a mat or flat sheet-like articles is preferred, the article having a thickness ranging from about 1 to 250 mils. By the proper control of fiber mixture composition and de position rate, a paper or sheet-like article can be made with a thickness ranging from about 1 to about mils. This paper can be made either impermeable or porous, as desired, the pores being in the range of l to 50 microns average diameter, as previously described. The

paper may be passed through calender rolls to give it a hard surface finish and increased mechanical strength.

It has been previously pointed out that the composition of the fibrous articles of the invention may be varied by changing the relative proportions of fusible and infusible fibers used in the fiber mixture. Mixtures of fusible and infusible novolac fibers may be employed, as well as mixtures of fusible and infusible fibers made from a pyrogenous mixture (or pitch) combined with a novolac. In addition to these two variations, fiber mixtures may be made in which infusible fibers of a pyrogenous mixture combined with a novolac are mixed with fusible novolac fibers. The relative proportions of fusible and infusible fibers in these three types of combinations, as well as the curing conditions, are all in the ranges as previously described. Experimental details for the preparation of fibrous articles,"using the three combinations described above, are shown in the following examples.

' EXAMPLE 1 Cured pitch-novolac fibers obtained as previously described were mixed with uncured pitch-novolac fibers, preferably produced from the same batch. The mixture of fibers was used to prepare a slurry according to the following procedure. Twelve grams of the cured fibers with an average diameter of l to 2 microns was put in 800 cc of water and 3 grams of uncured fiber was added. The mixture was thoroughly blended in a Waring blender for Ste 15 seconds to obtain a uniform slurry. It is necessary to control the mixing time, for if it is too long the fibers are broken down, and if it is too short a unifonn mixture is not obtained. In a separate vessel, 200 grams of hexamethylenetetramine was mixed with 800 cc of water and 15 cc of commercial grade phosphoric acid and the mixture was thoroughly blended. This mixture was then added to the fiber slurry made above. The overall composition of the slurry was: 1,500 cc of water; 12 grams of cured fiber; 3 grams of uncured fiber; 200 grams of hexamethylenetetramine; and 15 cc of phosphoric acid. The mix was allowed to stand for 10 to 12 hours. Commonly known surface active or wetting agents may be added to the mix to facilitate fiber wetting but in the present case, they were not essential.

A standard funnel (approximately 5 in. diameter) was cemented at the wide end with a fine 325 or 400 mesh screen. After the cement was dry, the small end of the funnel was connected to a vacuum pump, the funnel then put into the slurry and a layer of fibers collected on the screen mesh. Vacuum was applied for a period of 5 to 15 seconds to obtain a sheet or paper of appropriate thickness and density. The thickness and density of the sheet is defined mainly by l fiber diameter; (2) concentration of fiber in the slurry; and (3) extent of time for which vacuum is applied.

After the vacuum was shut off, an aqueous solution of PVA (polyvinylalcohol: 1 percent PVA in water) was sprayed onto the sheet or paper and suction ap plied again to remove excess water. When the mat or sheet of paper was firm, suction was released and a jet of air was used to strip the mat from the screen.

The sheet thus obtained was placed between two glass plates and heated on a hot plate (while occasionally reversing the sheet and glass plates on the hot plate) to about to C in 1 hour. The paper mat was then removed, put between two stainless steel plates, and heated from room temperature to about 370 C in 4 to hours. A load of 0.05 to 0.2 psi was applied to keep the paper uniformly flat. The paper when removed and tested in a flame produced no detectable odor or smoke.

The paper or mat prepared in the above fashion had a thickness in the range of 5 to 15 mils. When fibers of l to 2 microns were used as starting material, papers with areal densities in the range of 0.035 to 0.060 gms/sq in. with thicknesses of 5 to 15 mils were obtained. Areal density was obtained from the relation: Area density of Paper weight of paper/area of the paper The porosity of the papers was in the range of 80 to 85 percent. The porosity was computedas: percent porosity (lactual weight of material/theoretical weight of material) X 100 EXAMPLE 2 Cured pitch-novolac fibers of to microns average diameter obtained from methods described earlier were mixed with uncured fusible pitch-novolac fibers, preferably produced from the same batch. The fiber mixture was used to prepare a sheet or mat of paper as described in Example 1. The'paper prepared from the above fiber had a thickness in the range of l to 25 mils and the areal densities were in the range of 0.040 to 0.090 gms/sq in. when areal density was computed as in Example 1. The porosity values of the papers were in the range of 78 to 83 percent when the porosity was computed as in Example 1.

EXAMPLE 3 Cured pitch-novolac fibers of 10 to 15 microns diameter or 1 to 2 microns diameter, obtained from methods described earlier, were mixed with uncured fusible novolac fibers of similar diameter. The fiber mixture was used to prepare a sheet or mat of paper as described in Example 1. The paper prepared from the above fibers had a thickness in the range of 10 to 25 mils when the 10 to 15 micron diameter fibers were used or 5 to 15 mils with the 1 to 2 micron diameter fibers. The areal densities of the paper when computed as in Example. I, were in the range of 0.03 to 0.06 gms/sq in. with l to 2 micron diameter fibers, and 0.04 to 0.09 gms/sq in. with 10 to 15 micron diameter fibers, while the porosity was uniform and in the. range of 75 to 85 percent for papers obtained from either the large or small diameter fibers.

EXAMPLE 4 Novolac fibers of l to 2 microns diameter were obtained by blowing the fibers in a manner described earlier using novolac resin. The fibers were then cured in a hydrochloric acid and formaldehyde solution as described earlier. The cured fibers were mixed with uncured novolac fibers and a paper or sheet or mat was made as described inExample l. The paper was placed between the glass plates and heated on a hot plate (while occasionally reversing the paper and the glass plates on the hot plate) to 150 to 175 C in one hour.

The paper thus prepared had a thickness in the range of 5 to 15 mils, and had a uniform porosity in the range of 80 to 85 percent with an areal density of 0.04 to 0.08 gms/sq in.

The fibrous articles of the invention may be used as filters for solutions of acids and alkalis, since the fibers are resistant to these reagents. When made in paperlike sheets, these articles may be employed as separators in acid or alkaline batteries or as diaphragms in fuel cells. The fibers are electrically non-conductive and thus may be used in applications which require electrically insulative materials, the flexibility and mechanical strength of the fibrous sheet-like materials making them especially suited for this purpose. The thermal resistance of the fibrous articles of the invention allow them to function as thermal insulators at temperatures up to 500 C. It should be noted that the fiber mixture of the invention can be molded or otherwise formed in place around an object, the fiber mixture then being cured in place to give a bonded fibrous coating surrounding the object and protecting it against thermal and/or mechanical shock. If the fibrous covering was impervious, it could function as a safety envelope around a frangible container to prevent spillage of liquids held therein if the container was damaged or broken. It is apparent, therefore, that the fibrous articles of the invention fulfil the need for a flexible, po-

rous material which is chemically and thermally resistant and can be readily formed into mats or sheet-like shapes as required for industrial and scientific applications.

While the invention has been described with reference to certain examples and preferred embodiments, it is to be understood that various changes and modifications may be made by those skilled in art without departing from the broad spirit and scope of the present invention.

What is claimed is:

1. A process for making a bonded fibrous article from a mixture of fusible and infusible fibers, each selected from the group consisting of novolac fibers and fibers of a pyrogenous residue combined. with fromS to 40 wt percent of a novolac, said novolac in each instance having two or more available sites for polymerization, said process comprising;

a. Preparing a mixture comprising from about 10 percent to about 40 percent of fusible fibers and from about 60 percent to about 90 percent infusible fibers;

b. suspending the fibrous mixture in a fluid;

c. Depositing the fibers upon a permeable support;

d. Removing the fluid and adding a curing agent to the fibers to cure the fusible fibers; and

e. Compressing the fibers and simultaneously heating them to form an infusible bonded article.

2. A process according to claim 1 wherein said fusible fiber comprises a novolac.

3. A process according to claim 1 wherein said fusible fiber comprises said mixture of pyrogenous residue and novolac.

4. A process according to claim 1 wherein said mixture comprises about 20 percent fusible novolac fibers and about percent infusible fibers of said mixture of pyrogenous residue and novolac.

5. A process according to claim 1 wherein the curing agent is an aldehyde combined with a catalyst.

6. A process according to claim 1 wherein the curing agent is an amine combined with a catalyst.

7. A process according toclaim 1 wherein the fibers are compressed at a pressure less than 5 psi and heated to a temperature in the range of from about QQfErZQQ:

8. A process according to claim 7 wherein the heating step is followed by a second heating to a temperature range of from about 200 to 400 C.

9. A process according to claim I wherein the fiber mixture comprises about 20 percent fusible fibers and 80 percent infusible fibers, the mixture is suspended in water and deposited in a flat sheet upon a screen with the removal of water, followed by the addition of hexamethylenetetramine as a curing agent, the fibers being simultaneously compressed and heated at a temperature of from about 150 to 180 C, the pressure released and a second heating period continued at a temperature of from 200 to 370 C to form a bonded fibrous sheet.

10. A process for making an infusible fibrous sheet, comprising the steps of:

a. Preparing a mixture of from 10 to 40 percent of fusible fibers and from 60 to 90 percent of infusible fibers, said fusible and infusible fibers selected from novolacs and pyrogenous residues combined with from 5 to 40 wt percent of a novolac, said novolac in each instance having two or more available sites for polymerization, said fibers being from about 0.1 micron to about 300 microns in diameter;

b. Suspending said mixture in a fluid;

c. Depositing said mixture of fibers upon a permeable support;

d. Removing the fluid suspending agent from said fibers;

e. Contacting the fibers with a curing agent selected from the group consisting of aldehydes and amines, in the presence of a catalyst; and,

f. Compressing the fibers at a pressure less than 5 psi and simultaneously heating said fibers to a temperature of from about 90 to about 200 C for sufficient time to effect curing of the fusible fibers.

11. A process according to claim 10 wherein said fusible fibers comprise a novolac,

12. A process according to claim 10 wherein said fusible fibers comprise said mixture of pyrogenous residue and novolac.

13. A process according to claim 10 wherein said mixture comprises about 20 percent fusible novolac fibers and about percent infusible fibers of said mixture of pyrogenous residue and novolac.

14. A bonded fibrous article comprising infusible fibers selected from the group consisting of novolac fibers, fibers of a pyrogenous residue combined from 5-40 wt percent of a novolac, and mixtures thereof, said article prepared by suspending, in a fluid a mixture comprising from 10 to 40 percent fusible fibers and from 60 to percent infusible fibers, said fusible and infusible fibers each selected from the group consisting of novolac fibers and fibers of a pyrogenous residue combined with a novolac; depositing said mixture upon a substrate and removing said fluid; contacting said fibers with a catalyst and a curing agent selected from the group consisting of aldehydes and amines; and heating said fibers to a temperature of from 90 to 200 C under a pressure of less than 5 psi to cure said fusible fibers to an infusible state and to thereby bind said infusible fibers; 

2. A process according to claim 1 wherein said fusible fiber comprises a novolac.
 3. A process according to claim 1 wherein said fusible fiber comprises said mixture of pyrogenous residue and novolac.
 4. A process according to claim 1 wherein said mixture comprises about 20 percent fusible novolac fibers and about 80 percent infusible fibers of said mixture of pyrogenous residue and novolac.
 5. A process according to claim 1 wherein the curing agent is an aldehyde combined with a catalyst.
 6. A process according to claim 1 wherein the curing agent is an amine combined with a catalyst.
 7. A process according to claim 1 wherein the fibers are compressed at a pressure less than 5 psi and heated to a temperature in the range of from about 90* to 200* C.
 8. A process according to claim 7 wherein the heating step is followed by a second heating to a temperature range of from about 200* to 400* C.
 9. A process according to claim 1 wherein the fiber mixture comprises about 20 percent fusible fibers and 80 percent infusible fibers, the mixture is suspended in water and deposited in a flat sheet upon a screen with the removal of water, followed by the addition of hexamethylenetetramine as a curing agent, the fibers being simultaneously compressed and heated at a temperature of from about 150* to 180* C, the pressure released and a second heating period continued at a temperature of from 200* to 370* C to form a bonded fibrous sheet.
 10. A process for making an infusible fibrous sheet, comprising the steps of: a. Preparing a mixture of from 10 to 40 percent of fusible fibers and from 60 to 90 percent of infusible fibers, said fusible and infusible fibers selected from novolacs and pyrogenous residues combined with from 5 to 40 wt percent of a novolac, said novolac in each instance having two or more available sites for polymerization, said fibers being from about 0.1 micron to about 300 microns in diameter; b. Suspending said mixture in a fluid; c. Depositing said mixture of fibers upon a permeable support; d. Removing the fluid suspending agent from said fibers; e. Contacting the fibers with a curing agent selected from the group consisting of aldehydes and amines, in the presence of a catalyst; and, f. Compressing the fibers at a pressure less than 5 psi and simultaneously heating said fibers to a temperature of from about 90* to about 200* C for sufficient time to effect curing of the fusible fibers.
 11. A process according to claim 10 wherein said fusible fibers comprise a novolac.
 12. A process according to claim 10 wherein said fusible fibers comprise said mixture of pyrogenous residue and novolac.
 13. A process according to claim 10 wherein said mixture comprises about 20 percent fusible novolac fibers and about 80 percent infusible fibers of said mixture of pyrogenous residue and novolac.
 14. A bonded fibrous article comprising infusible fibers selected from the group consisting of novolac fibers, fibers of a pyrogenous residue combined from 5-40 wt percent of a novolac, and mixtures thereof, said article prepared by suspending, in a fluid a mixture comprising from 10 to 40 percent fusible fibers and from 60 to 90 percent infusible fibers, said fusible and infusible fibers each selected from the group consisting of novolac fibers and fibers of a pyrogenous residue combined with a novolac; depositing said mixture upon a substrate and removing said fluid; contacting said fibers with a catalyst and a curing agent selected from the group consisting of aldehydes and amines; and heating said fibers to a temperature of from 90* to 200* C under a pressure of less than 5 psi to cure said fusible fibers to an infusible state and to thereby bind said infusible fibers. 